We thank Erlwein et al. (1) and Martin (2) for their comments on our recent publication (3). As both letters raise some similar issues, we will address all of the comments here in one response.

We were aware that the SuperScript III One-Step RT-PCR System (cat. no. 12574–030; Invitrogen) had come under question for contamination with mouse DNA. For that reason, we rigorously examined the Platinum Taq polymerase we used (cat. no. 10966–034, lot 727463; Invitrogen) for any possible mouse DNA contamination. First, as stated in the article (3), no murine leukemia virus (MLV)-related gene amplicon was produced in more than 300 negative controls tested in parallel. Second, we tested the polymerase and our PCR assay system repeatedly using the highly sensitive mouse DNA-specific seminested PCR targeting the mitochondria DNA described; we never detected any mouse DNA contamination.

As for the “nonspecific” side bands seen in the gel, we have found that virus gag gene-specific primer sets could sometimes amplify human DNA nonspecifically and produce amplicons of different sizes. Thus, Martin was correct that any murine retroviral sequences would need to compete for the primers with the nonspecific reactions also occurring with normal human DNA. This would likely lower …